1. Field of the Invention
The present invention relates to a structure of a metal substrate installed in an exhaust system of a vehicle.
2. Description of the Related Art
In the exhaust system of the vehicle, a metal substrate for carrying a catalyst for oxidizing or reducing the harmful components in an exhaust gas is installed. Many metal substrates have been up to now proposed in which a large corrugated foil having relatively large corrugations and a small corrugated foil having small corrugations are overlapped and wound alternately.
This metal substrate has a number of cells formed between each corrugated foil to keep a contact area with the exhaust gas. The corrugated foil may be typically made from a foil material of 20Crxe2x80x945Alxe2x80x94Fe having a thickness of 30 to 50 xcexcm in consideration of resistance to oxidation. The corrugated foils are connected by brazing of nickel (Ni) so that the cell density is 600 cells/square inches at maximum.
However, in the metal substrate according to the related art, since the corrugated foils are connected by brazing of nickel, base materials Al and Cr are diffused into a fillet of nickel brazing material, while the brazing material Ni is immersed into the base materials. Therefore, Al and Cr in the foil material of corrugated foils are consequently diluted, resulting in a problem that the resistance to oxidation is decreased.
Along with an enhanced requirement performance of exhaust emission purification, there is need for increasing the contact area with the exhaust gas on a carrying surface of catalyst by increasing the cell density in the metal substrate in which the corrugated foils are overlapped and wound as described above. However, if the cell density was further increased using the materials according to the related art, it was found that the expected exhaust emission purification effect could not be obtained at the low temperatures at the early stage of starting the operation.
The reason is that rise in temperature over the entire area of the metal substrate is slower than conventionally. That is, with the higher cell density, the surface area is increased, but the number of winding the corrugated foil is also increased, so that the mass and heat capacity thereof are increased to delay the rise in temperature over the entire area of the metal substrate.
In the light of the above-mentioned problems, it is an object of the present invention to provide a metal substrate in which the exhaust emission purification performance is further improved by making the cell density higher while suppressing the heat capacity to be low.
According to a first aspect of the invention, there is provided a metal substrate having an outer casing and a honeycomb structure formed by overlapping and winding a large corrugated foil and a small corrugated foil, the honeycomb structure held in the outer casing. The large corrugated foil and the small corrugated foil have thickness in a range of 18 xcexcm to 22 xcexcm. The honeycomb structure has cell density in a range of 810 cells/square inches to 990 cells/square inches.
According to a second aspect of the invention, the large corrugated foil and the small corrugated foil in the honeycomb structure are connected by diffused junction.
According to a third aspect of the invention, the large corrugated foil has a wave height in a range of 0.76 mm to 0.86 mm and a pitch in a range of 1.66 mm to 1.76 mm. The small corrugated foil has a wave height in a range of 0.05 mm to 0.18 mm and a pitch in a range of 1.29 mm to 1.39 mm.
According to a fourth aspect of the invention, the small corrugated foil is overlapped and wound around an outermost circumference of the honeycomb structure multiple times.
According to a fifth aspect of the invention, the honeycomb structure is brazed with an inner circumferential face of the outer casing by a brazing foil disposed in an area spaced away a predetermined distance from a back end face of the honeycomb structure on a rear-half side in a flow direction of exhaust gas.
According to a sixth aspect of the invention, the outer casing is formed with a radially outwardly swollen bead around the entire circumference of the outer casing on an opposite side to the rear end face of the honeycomb structure, with the brazing foil before brazing sandwiched.